WO2000074420A1

WO2000074420A1 - Method and arrangement for switching cells

Info

Publication number: WO2000074420A1
Application number: PCT/FI1999/000722
Authority: WO
Inventors: Jari VALLSTRÖM; Pekka Möttönen; Kari JYRKKÄ; Harri Jokinen
Original assignee: Nokia Corporation
Priority date: 1999-06-01
Filing date: 1999-09-06
Publication date: 2000-12-07
Also published as: WO2000074423A1; US6804212B1; NO20015675D0; SE530260C2; CN1145403C; FI112841B; FI19991853A; AU4762300A; NO20015675L; CN1361999A; RU2237363C2; SE0104027L; AU5520399A

Abstract

The invention pertains to a method and arrangement for switching the serving base station. The invention is especially applicable to digital TDMA (Time Division Multiple Access) cellular systems in which the operating ranges of the cells are wide. An idea of the invention is that data transmission proper utilizes extended timing advance, and the mobile station sends to the new base station a HANDOVER ACCESS message on a logical channel established for such a message in order to start the handover. A second idea of the invention is to use call re-establishment procedure in a handover. This feature can be used even in cells/systems without an extended range operation. A third idea of the invention is that in a synchronous handover the mobile station sends the handover request to the base station on a DCCH channel using a timing advance. A fourth idea of the invention is that the mobile station sends the handover request to the base station on a DCCH channel, and the base station barres the consecutive slot from traffic during the handover, but allows a time slot for traffic, if a handover access burst is not expected to extend on the time slot. By means of the invention it is possible to create cells with wide operating ranges using only one GSM carrier, whereby it is possible to carry out a handover to the appropriate cell. Furthermore, the solution according to the invention makes it possible to realize extended cells with no significant reduction in the cell's data transmission capacity.

Description

Method and arrangement for switching cells

The invention relates to a method and arrangement for switching the serving base station. The invention finds particular utility in digital TDMA (Time Division Multiple Access) cellular systems in which the operating ranges of cells are wide. The 450-MHz GSM (Global System for Mobile Telecommunications) is one such cellular system.

A mobile station in a cellular radio system always attempts to select a base station on the coverage area, or cell, of which it will camp. Conventionally the cell selection has been based on measuring the strength of the received radio signal at either the mobile station or base station. In the GSM, for example, each base station sends out a signal on a so-called Broadcast Control Channel (BCCH) the frequencies of which are different at adjacent base stations, and the mobile stations measure the strengths of the BCCH signals received, and decide on the basis of the measured strengths which cell is the most advantageous one in regard to the radio link quality. The base stations also send to the mobile stations information about the BCCH frequencies used in the neighboring cells so that the mobile stations know what frequencies they have to listen to in order to find the BCCH transmissions of the neighboring cells. In each cell the BCCH transmission also includes information about how the mobile stations can make so-called random access requests in that particular cell in order to establish connections for calls.

In TDMA systems, information is usually transmitted in time slots of TDMA frames at a certain operating frequency. In order for the bursts sent by the mobile station to be timed correctly so as to find the right time slot the transmission moment of the mobile station must be determined also on the basis of the distance between the mobile station and base station in such a manner that the propagation delay of the burst provides for a timing advance for the mobile station's transmis- sion time. For example, in the GSM the maximum timing advance is 233 μs, corresponding to a 35-km propagation delay. The mobile station's timing advance parameter may take values from 0 to 63, where 63 corresponds to the maximum timing advance 233 μs.

Fig. 1 is a flow diagram depicting a method according to the GSM for switching the serving base station. First, the current serving base station sends a HANDOVER COMMAND message to a mobile station, step 10. This message indicates the new serving base station. Then the mobile station sends to the new base station HANDOVER ACCESS bursts, step 12. These random access type bursts are transmitted on a dedicated control channel (DCCH) allotted to the mobile station. The DCCH uses the same TDMA frame time slot as the traffic channel TCH. Having received the HANDOVER ACCESS message the new base station determines a timing advance for the mobile station and sends it and other related information to the mobile station in a PHYSICAL INFORMATION message, step 14. If the handover is synchronous, however, the mobile station already knows the timing advance. Finally, the mobile station sends to the new serving base station a HANDOVER COMPLETE message, whereafter the old base station no longer serves the mobile station.

In some cases it has become necessary in TDMA systems to employ so-called extended cells in which the operating range could be over 35 km. One reason for this need is the introduction of lower frequencies. For example, a 450-MHz GSM system could operate at a distance of up to 120 km from base station as far as the allowable transmission power is concerned. However, problems are then caused by the fact that the maximum timing advance is not sufficient to compensate for the propagation delay of the signal.

Fig. 2 shows two base stations BTS1 201 and BTS2 202 of a cellular system according to the prior art and their operating ranges. Cells 202 and 212 of the base stations are conventional cells in which the maximum operating range is 30 km from base station. At that distance the timing advance is 54. Cells 203 and 213 of the base stations are conventional cells, too, with a maximum operating range of 35 km from base station, in which case the timing advance is 63. As said, this timing advance is the maximum timing advance in a conventional GSM system. Cells 204 and 214 are extended cells in which the maximum operating range from base station is 120 km. The maximum timing advance parameter required in this case is 215.

Fig. 2 also shows a mobile station MS 120. When the mobile station is moving in the direction of the arrow it must switch from BTSl to BTS2. If the system allows for timing advance parameters greater than 63, the problem is how to send a random access message for a handover from the mobile station to base station if the mobile station does not know the correct timing advance parameter value. Normally the mobile station sends the random access request with a timing advance parameter value 0. If, however, the correct timing advance parameter value were greater than 63, the random access request would not arrive at the base station within the time window reserved for the message.

One way of avoiding the problem is to use only every other time slot for data transmission, which makes possible operation at a distance corresponding to a propagation delay of 580 μs. A drawback of this solution is, however, that it cuts the transmission capacity reserved for the traffic channels in half.

An object of the invention is to provide a solution for switching the serving cell, avoiding the above-described disadvantages associated with the prior art. Especially, with the present inventive idea it is possible to avoid using continuous barring of every other time slot from traffic and thus it is possible to achieve a higher transmission capacity in the cells with extended oerating range.

An idea of the invention is that data transmission proper utilizes extended timing advance, and the mobile station sends to the new base station a HANDOVER ACCESS message on a logical channel established for such a message in order to start the handover.

According to an embodiment of the invention, such a logical channel can be established on the uplink common control channel CCCH in such a manner that two logical channels are created, one of which is used to transfer the CHANNEL REQUEST messages associated with the call set-up, and the other is used to transfer the HANDOVER ACCESS messages.

According to a second embodiment of the invention, a logical channel is established on the uplink common control channel CCCH in such a manner that in connection with the handover CHANNEL REQUEST messages are used which include a special identifier field, establishment cause, for example.

A second idea of the invention is that call re-establishment procedure is used in the handover. Then the access burst may be a CHANNEL REQUEST message where a specified identifier, eg. establishment cause, is set to "call re- establishment". This procedure is similar to the procedure of re-establishing an unintentionally disconnected connection between a base station and a mobile station.

A third idea of the invention is that in synchronous handover the mobile station sends the handover request to the base station on DCCH using a timing advance.

A fourth idea of the invention is that the mobile station sends the handover request to the base station on DCCH, and the base station barres the time slot following the DCCH time slot from traffic when the handover is taking place. If a handover to a certain time slot at a certain time is not probable, the time slot following said certain time slot is not barred from traffic.

By means of the invention it is possible to create cells with wide operating ranges using only one GSM carrier, whereby it is possible to carry out a handover to the appropriate cell. Furthermore, the solution according to the invention makes it possible to realize extended cells with no significant reduction in the cells' data transmission capacity. By means of the solution a conventional system can in a simple manner be turned into a system allowing for extended cells.

The method according to the invention for realizing a handover in a time-division cellular network utilizing extended timing advance in data transmission wherein the mobile station requests a handover by sending to the new base station a handover message (HANDOVER ACCESS) is characterized in that the handover message is sent on a logical channel established for that purpose.

The invention also pertains to a method for switching the serving base station in a time-division cellular network utilizing extended timing advance in data transmission wherein a mobile station sends to the new base station a handover message (HANDOVER ACCESS), characterized in that the handover message is a call re-establishment message.

In addition the invention pertains to a method for switching serving base stations in a time-division cellular network utilizing extended timing advance in data transmission wherein the mobile station requests a handover by sending to the new base station a handover request (HANDOVER ACCESS), characterized in that the handover request is sent on a control channel (DCCH) allotted to the mobile station, using timing advance.

The invention further pertains to a method for switching the serving base station in a time-division cellular network utilizing extended timing advance in data transmission wherein a mobile station requests a handover by sending to the new base station a handover request (HANDOVER ACCESS) on a time slot of a control channel dedicated to the mobile station, characterized in that when the handover request is transmitted, the time slot next following time slot of the dedicated control channel is barred from traffic, and at a time when a handover to a determined time slot is not probable, the time slot next following said determined time slot is allowed for traffic.

The cellular system according to the invention, which comprises means for switching the serving base station in a time-division cellular network in which extended timing advance is used in the data transmission between base station and mobile station and where the base station comprises means for receiving a handover message (HANDOVER ACCESS) from the mobile station, is characterized in that the base station comprises means for receiving a handover message (HANDOVER ACCESS) from the mobile station on a logical channel established for that purpose.

The invention also pertains to a cellular system comprising means for switching the serving base station in a time-division cellular network such that the base station comprises means for receiving a handover message (HANDOVER ACCESS) from the mobile station, characterized in that the cellular system comprises means for switching the serving base station with a re-establishment procedure.

The invention further pertains to a cellular system comprising means for switching the serving base station in a time-division cellular network such that the base station comprises means for receiving a handover request message (HANDOVER ACCESS) on a time slot of a control channel (DCCH) dedicated to the mobile station, characterized in the cellular system comprises means for barring from traffic the time slot next following the time slot of the dedicated control channel when the handover request message is transmitted, and at a time when a handover to a determined time slot is not probable, allowing for traffic the time slot next following said determined time slot.

The mobile station according to the invention, comprising means for switching the serving base station in a time-division cellular network, means for using extended timing advance in the data transmission between the mobile station and base station, and means for sending a handover message (HANDOVER ACCESS) to the new base station, is characterized in that the mobile station comprises means for sending a handover message on a logical channel established for that purpose.

The invention also pertains to a mobile station comprising means for switching the serving base station in a time-division cellular network, and means for sending a handover message (HANDOVER ACCESS) to the new base station, characterized in that the mobile station comprises means for using a call re- establishment access message as a handover message.

The invention further pertains to a mobile station that comprises means for switching the serving base station in a time-division cellular network, means for using extended timing advance in the data transmission between the mobile station and base station, and means for sending a handover message (HANDOVER ACCESS) to the new base station, characterized in that the mobile station comprises means for sending a handover request on a control channel (DCCH) allotted to the mobile station, using timing advance.

Preferred embodiments of the invention are disclosed in the independent claims.

Extended timing advance means in this patent application that the maximum value of the timing advance used corresponds to the propagation delay which shifts the access burst outside the time slot window.

The invention is below described in greater detail referring to the accompanying drawings in which

Fig. 1 shows conventional and extended cells in a GSM system,

Fig. 2 is a flow diagram illustrating a known method for switching serving base stations in a GSM system, Fig. 3 illustrates the use of channels when switching serving base stations in a method according to the invention, Fig. 4a is a flow diagram illustrating a method according to the invention for switching serving base stations in an asynchronous system,

Fig. 4b is a flow diagram illustrating a method according to the invention for switching serving base stations with call re-establishment procedure, Fig. 5a is a flow diagram illustrating a method according to the invention for switching serving base stations in a synchronous system, Fig. 5b is a flow diagram illustrating a method according to the invention for switching serving base stations where a consecutive time slot is barred during a handover, Fig. 6 shows a mobile station and cellular system according to the invention.

Figs. 1 and 2 were already discussed in connection with the description of the prior art.

Fig. 3 illustrates a method according to the invention for using channels to switch serving base stations. The figure shows a TDMA frame in cell 1, 310, comprising time slots 311-318. The logical control channel RACH/HOCH according to the invention is located in time slot 311. In the case of an extended cell the second time slot 312 in the TDMA frame is not used as a traffic channel. The traffic channel of the mobile station MS is located in time slot 315 in this cell.

Let us examine a situation in which cell 2, 320, is selected as the new serving cell. In this case the extended cell comprises TDMA time slots 321-328. The mobile station sends a HANDOVER ACCESS burst to the common control channel RACH/HOCH of cell 2. This is depicted by arrow 331. When the mobile station after that learns the correct timing advance of cell 2, it can transmit on the traffic channel TCH allotted to it, located in this case in time slot 325. This is depicted by arrow 332.

Fig. 4a is a flow diagram illustrating a method according to the invention for switching serving base stations asynchronously.

A System Information message sent on the broadcast control channel BCCH includes information about whether the base station in question supports extended cells or not. This information can be included e.g. in the "Cell Options" field in the SYSTEM INFORMATION TYPE message. This way, the mobile station can perform measurements, if necessary, on the BCCH of a base station that is known to support extended cells, step 45.

The current serving base station sends a HANDOVER COMMAND message, step 46, which indicates that the new cell supports the extended operating range feature. This new information can be included e.g. in the Channel Description 2 information element in the message. Thus the mobile station knows that the burst of the HANDOVER ACCESS message should be sent in the appropriate time slot of the common control channel CCCH, step 47.

After that, the mobile station monitors the DCCH in order to receive a PHYSICAL INFORMATION message. Once the new base station has sent to the mobile station the PHYSICAL INFORMATION message, indicating, among other things, the correct timing advance parameter value to the mobile station, step 48, the mobile station sends to the new base station a HANDOVER COMPLETE message. After that, the connection is served by the new base station. After a successful handover the timing advance parameter value is updated as usual.

Instead of the PHYSICAL INFORMATION message sent on the DCCH, an IMMEDIATE ASSIGNMENT message can be sent to the mobile station on the

CCCH, indicating the new parameter value for the timing advance that should be used by the mobile station on the channel allotted to it. The IMMEDIATE ASSIGNMENT message also includes the random reference number used by the mobile station in the CHANNEL REQUEST message.

It is the responsibility of the base station to coordinate the sending of the HANDOVER ACCESS message from the mobile station as well as the transfer of the response on the CCCH or new DCCH.

A logical channel can be established on the CCCH in one of the following two ways, for example.

In accordance with a first embodiment, the CCCH is divided in the uplink direction between two separate logical channels comprising either the conventional CHANNEL REQUEST message or the HANDOVER ACCESS message. The different messages cannot be used simultaneously.

In accordance with a second embodiment, the mobile station should send the HANDOVER ACCESS message using the conventional CHANNEL REQUEST message as the random access burst, but now with a new handover identifier field, such as establishment cause, for example. It can be indicated in several alternative ways originally reserved for future needs of the CHANNEL REQUEST message.

Fiure 4b illustrates one further idea according to the invention where a so-called call re-establishment procedure is used in a handover. This is a similar procedure to what is used when a mobile station unintentionally loses connection to the base station. The procedure follows similar phases as described in figure 4a, but in the call re-establishment procedure a CHANNEL REQUEST message (or RE- ESTABLISHMENT REQUEST) is used as a HANDOVER ACCESS message. The CHANNEL REQUEST message has in this case a "call re-establishment" identifier field, such as an establishment cause. A System Information message sent on the broadcast control channel BCCH includes information about whether the base station in question supports extended cells or not. This information can be included e.g. in the "Cell Options" field in the SYSTEM INFORMATION TYPE message. This way, the mobile station can perform measurements, if necessary, on the BCCH of a base station that is known to support extended cells, step 45.

The current serving base station sends a HANDOVER COMMAND message, step 46, which may indicate, that the new cell supports the extended operating range feature, and also that call re-establishment procedure is used in the handover. This information can be included e.g. in the Channel Description 2 information element in the message. Thus the mobile station knows that the burst of the CHANNEL REQUEST message with "call re-establishment" identifier field should be sent in the appropriate time slot of the common control channel CCCH (step 47).

After that, the mobile station monitors the CCCH in order to receive a IMMEDIATE ASSIGNMENT message. Once the new base station has sent to the mobile station the IMMEDIATE ASSIGNMENT message, indicating, among other things, the correct timing advance parameter value to the mobile station and the random reference number used by the mobile station, step 48, the mobile station sends to the new base station a HANDOVER COMPLETE message. After that, the connection is served by the new base station. After a successful handover the timing advance parameter value is updated as usual.

Instead of the IMMEDIATE ASSIGNMENT message sent on the CCCH, a PHYSICAL INFORMATION message can be sent to the mobile station on the DCCH, indicating the new parameter value for the timing advance that should be used by the mobile station on the channel allotted to it.

It should be noted that the call re-establishment can be applied in handovers even if the handover would not be executed to an extended operating range, and even if the base station would not support extended operating ranges. The call re- establishment procedure can be used in both synchronous and asynchronous handovers.

Fig. 5a is a flow diagram illustrating a method according to the invention for performing a synchronous handover using a timing advance in the access burst.

A System Information message sent on the broadcast control channel BCCH includes information about whether the base station in question supports extended cells or not. This information can be included e.g. in the "Cell Options" field in the SYSTEM INFORMATION TYPE message. This way, the mobile station can perform measurements, if necessary, on the BCCH of a base station that is known to support extended cells, step 55.

To initiate a handover the current base station sends to the mobile station a HANDOVER COMMAND message, step 56. This message includes information about the new serving base station and whether the new cell supports extended operating range or not. This new information can be included e.g. in the Channel Description 2 information element. After that, the mobile station sends HANDOVER ACCESS bursts to the new base station, step 57. These random access type bursts are sent on the dedicated control channel DCCH allotted to the mobile station. In a synchronous handover the mobile station already knows the new timing advance parameter value, which is used in the sending of the HANDOVER ACCESS message on the DCCH. After that, there may be a PHYSICAL INFORMATION message from the new base station, and the mobile station then starts normal transmission using the new timing advance and sends to the new base station a HANDOVER COMPLETE message, after which the old base station no longer serves the mobile station.

Figure 5b illustrates a flow diagram of a further idea according to the present invention. In this embodiment the mobile station sends the handover request to the base station on a DCCH channel, and the base station barres the consecutive slot from traffic during the handover, but allows a time slot for traffic, if a handover access burst is not expected to exted on the time slot.

When the mobile station approaches the area where a handover is necessary, the new base station receives information on a probable handover. This information comes from the current base station / base station system. Then the new base station reserves a time slot for an uplink DCCH channel and barres from traffic the next time slot following the reserved DCCH time slot, phase 551.

To initiate a handover the current base station sends to the mobile station a HANDOVER COMMAND message, step 56. This message includes information about the new serving base station and whether the new cell supports extended operating range or not. This new information can be included e.g. in the Channel Description 2 information element. After that, the mobile station sends HANDOVER ACCESS bursts to the new base station, step 57. These random access type bursts are sent on the dedicated control channel DCCH reserved for the mobile station.

After that, there may be an IMMEDIATE ASSIGNMENT message from the new base station, and the mobile station then starts normal transmission using the new timing advance and may send to the new base station a HANDOVER

COMPLETE message, after which the old base station no longer serves the mobile station, step 58.

After the handover is completed, the new base station may allow the barred time slot for traffic again. So, according to the invention it is possible to use barring for a short time when the access burst are transmitted. It is also possible to select the time slot for a DCCH channel in such a way that there is a free consecutive time slot. It should also be noted that barring is not necessarywhen the mobile station is within the normal operating distance from the base station. So in this way barring the time slots due to the handover does not cause a significant decrease in the transmission capacity of the extended cells.

Fig. 6 shows a simplified block diagram of a mobile station 600 according to the invention and its connection to a cellular telephone network. The mobile station comprises an antenna 601 for receiving radio-frequency (RF) signals transmitted by base stations. A received RF signal is directed by a switch 602 to a RF receiver 611 where the signal is amplified and converted digital. The signal is then detected and demodulated in block 612. Block 613 performs decryption and deinter leaving. Then follows signal processing in block 630. Received data may be stored as such in the mobile station's memory 604 or, alternatively, the processed packet data are taken after the signal processing to a possibly external device such as a computer. A control unit 603 controls the above-mentioned reception blocks in accordance with a program stored in the unit.

Transmission from the mobile station is performed e.g. as follows. Controlled by the control block 603, block 633 performs possible signal processing on the data and block 621 performs interleaving and encryption on the processed signal to be transmitted. Bursts are generated from the encoded data, block 622, which are modulated and amplified into a RF signal to be transmitted, block 623. The RF signal to be transmitted is lead to an antenna 601 through a switch 602. The processing and transmission functions described above are controlled by the control unit 603.

In the mobile station of Fig. 6, the components that are essential from the invention's perspective include the prior-art reception blocks 611-613, by means of which the mobile station receives, demodulates and decodes the messages transmitted by base stations, as well as the control block 603 which processes the information contained in the messages and controls the operation of the mobile station. Part of the memory 604 of the mobile station can be allocated to information relating to extended cells in accordance with the invention. The mobile station uses the transmission blocks 621-623 to transmit the handover messages according to the invention to base stations.

Fig. 6 also shows how the mobile station is connected to the cellular system. The cellular system comprises a plurality of base stations, 651, which send and receive RF signals through antennas 650. One or more base stations are controlled by a base station controller 652. Several base station controllers are connected to a mobile switching center 653. The mobile switching center may be connected to other mobile switching centers and other data transmission systems.

The hardware requirements caused by the invention on base stations and mobile stations are not great. Base station 651 and/or base station controller 652 have access to a database (not shown in Fig. 6) containing information about extended cells. An operator usually has operation and maintenance (OM) capabilities for altering the contents of such databases statically (e.g. when a new base station starts operation in the vicinity of existing base stations) or dynamically. Generation and transmission of messages associated with information contained in the database can be realized using prior-art means 651 , 652 programmed so as to carry out signaling and channel allocation according to the invention.

The embodiments described above are exemplary only and do not limit the application of the invention. Particularly it should be noted that although the examples described above are related to the GSM system, the invention is applicable to any other digital time-division cellular system. Especially the general packet radio service (GPRS) and high-speed circuit-switched data (HSCSD) service associated with the GSM system could be mentioned as examples. It should also be noted that the names used for channels or messages serve only as examples and the invention is not in any way restricted to them.

Claims

1. A method for switching the serving base station in a time-division cellular network utilizing extended timing advance in data transmission wherein a mobile station sends to the new base station a handover message (HANDOVER ACCESS), characterized in that the handover message is sent on a logical channel established for that purpose.

2. A method according to claim 1, characterized in that said logical channel is established on an uplink common control channel (CCCH).

3. A method according to claim 2, characterized in that on the common control channel (CCCH) there is established a first logical channel for the transfer of handover messages (HANDOVER ACCESS) and a second logical channel for connection requests (CHANNEL REQUEST).

4. A method according to claim 3, characterized in that said logical channels are mutually not coincident in time.

5. A method according to claim 2, characterized in thai said logical channel is established on the common control channel by using for the transfer of handover messages a connection request message (CHANNEL REQUEST) that includes an identifier field (establishment cause) referring to a handover.

6. A method according to any one of claims 1 to 5, characterized in that the handover is asynchronous.

7. A method according to any one of claims 1 to 6, characterized in that the current base station sends to the mobile station a handover command (HANDOVER COMMAND) including information about the possibility of using the extended cell feature in the new serving cell.

8. A method for switching the serving base station in a time-division cellular network wherein a mobile station sends to the new base station a handover message (HANDOVER ACCESS), characterized in that the handover message is a call re-establishment message.

9. A method according to claim 8, characterized in that said handover message is a connection request message (CHANNEL REQUEST) that includes an identifier field (establishment cause) referring to call re-establishment.

10. A method according to claim 8 or 9, characterized in that the current base station sends to the mobile station a handover command (HANDOVER COMMAND) including information about the possibility of using the extended cell feature in the new serving cell.

11. A method for switching the serving base station in a time-division cellular network utilizing extended timing advance in data transmission wherein a mobile station requests a handover by sending to the new base station a handover request (HANDOVER ACCESS), characterized in that the handover request is sent on a control channel (DCCH) allotted to the mobile station, using a timing advance.

12. A method for switching the serving base station in a time-division cellular network utilizing extended timing advance in data transmission wherein a mobile station requests a handover by sending to the new base station a handover request (HANDOVER ACCESS) on a control channel (DCCH) time slot dedicated to the mobile station, characterized in that when the handover request is transmitted, the time slot next following time slot of the dedicated control channel is barred from traffic, and at a time when a handover to a determined time slot is not probable, the time slot next following said determined time slot is allowed for traffic.

13. A method according to claim 12, characterized in that the probability for a handover is determined and informed to a probable new base station, and said information is used for barring/allowing a time slot from/for traffic.

14. A method according to claim 12, characterized in that the new base station selects the dedicated control channel for a mobile station executing a handover to be a time slot where the the next following time slot is free from traffic.

15. A cellular system comprising means for switching the serving base station in a time-division cellular network utilizing extended timing advance in the data transmission between base station and mobile station such that the base station comprises means for receiving a handover message (HANDOVER ACCESS) from the mobile station, characterized in that the base station comprises means for receiving a handover message (HANDOVER ACCESS) from the mobile station on a logical channel established for that purpose.

16. A cellular system comprising means for switching the serving base station in a time-division cellular network such that the base station comprises means for receiving a handover message (HANDOVER ACCESS) from the mobile station, characterized in that the cellular system comprises means for switching the serving base station with a re-establishment procedure.

17. A cellular system comprising means for switching the serving base station in a time-division cellular network such that the base station comprises means for receiving a handover request message (HANDOVER ACCESS) on a time slot of a control channel (DCCH) dedicated to the mobile station, characterized in the system comprises means for barring from traffic the time slot next following the time slot of the dedicated control channel when the handover request message is transmitted, and at a time when a handover to a determined time slot is not probable, allowing for traffic the time slot next following said determined time slot.

18. A mobile station comprising means for switching the serving base station in a time-division cellular network, means for utilizing extended timing advance in the data transmission between the mobile station and base station, and means for sending a handover message (HANDOVER ACCESS) to the new base station, characterized in that the mobile station comprises means for sending a handover message on a logical channel established for that purpose.

19. A mobile station comprising means for switching the serving base station in a time-division cellular network, and means for sending a handover message (HANDOVER ACCESS) to the new base station, characterized in that the mobile station comprises means for using a call re-establishment access message as a handover message.

20. A mobile station comprising means for switching the serving base station in a time-division cellular network, means for utilizing extended timing advance in the data transmission between the mobile station and base station, and means for sending a handover message (HANDOVER ACCESS) to the new base station, characterized in that the mobile station comprises means for sending a handover message on a control channel (DCCH) allotted to the mobile station, using a timing advance.